In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot combustion gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
Several critical components of commercial and military gas turbine engines are manufactured from alpha-beta titanium alloys. Examples of such components include fan disks and compressor disks. These components support the respective turbine and compressor blades and rotate at high speeds about their shafts during service of the gas turbine engine.
The fan and compressor disks are typically prepared by melting the titanium alloy of the appropriate composition, casting the titanium alloy as an ingot, and converting the ingot to the billet form. The starting ingot may be as much as 30 inches thick, or more in some circumstances. The billet is mechanically converted by forging to smaller thicknesses and finally forged by closed-die forging to produce the fan or compressor disk in a nearly final form, which is then heat treated and final machined. The fan and compressor disks in their final form may be as large as 40 inches or more in diameter, and as much as 6 inches or more thick, for large-size gas turbine engines.
Some of the important mechanical properties of the large-size forged articles are not as good as those obtained in similarly processed small-size forged or otherwise fabricated articles. For example, in one test the fatigue run-out stress of a 6-inch thick forging is about 23 ksi, and the fatigue run-out stress of a 1.75-inch diameter bar is about 36 ksi. It is therefore necessary to design the large forged article, such as the fan or compressor disk, larger and heavier than would be required if the same fatigue properties achieved in the smaller article could be achieved in the larger article.
The disparity in properties results from the inability to achieve the same fine-scale microstructure throughout the thick forging as is achieved in the smaller bar. That is, the processing of thick articles is qualitatively different from the processing of thinner articles, because of several factors. For example, the center of a thick article cannot be heated as rapidly in an oven or cooled as rapidly in quenching, as can the periphery of the thick article or the entirety of a thin article. The metal flow at the center of the thick article is not as great as that at the periphery of the thick article or throughout the entirety of the thin article. The total amount of reduction is also different for the two sizes. There may be compositional and microstructural gradients through the thick article. Consequently, many of the properties that are readily achieved in thin, essentially uniform articles cannot be achieved in thicker articles.
This problem has long been recognized, and various attempts have been made to improve the properties of thick articles. A surface treatment may be used to improve the properties such as fatigue resistance. The thick article may be fabricated as two or more smaller articles and then joined together. Different alloys may be used in which the thickness-dependence of properties is less. All of these approaches are costly to implement, impossible to apply in some circumstances of limited access and the like, and in some cases introduce their own new problems to be overcome.
There remains a need for an approach to producing thick articles of alpha-beta titanium alloys in which the structures and properties achieved are more nearly like those attained in thin articles. The present invention fulfills this need, and further provides related advantages.